1. Field of the Invention
The present invention relates to an optical encoder that optically reads a periodic pattern provided on a scale to output, a signal corresponding to the periodic pattern.
2. Description of the Related Art
Optical encoders are constituted by a scale provided with a periodic pattern and a sensor movable relatively with the scale. The sensor as a light receiver photoelectrically converts a detection light emitted from a light source and then passing through optical portions that reflects or transmits the defection light to output a signal (hereinafter referred to as “a periodic signal”) having a change period corresponding to the periodic pattern. Use of the periodic signal enables detection of a relative movement amount (or a relative position) of the scale and the sensor and detection of an absolute position of one of the scale and the sensor. Furthermore, interpolation of the periodic signal enables position detection with, a finer resolution than a period of the periodic pattern. The period means a spatial period or a cycle length.
However, the periodic signal contains higher-order components that are integral multiple periodic components of the period of the periodic pattern, which makes it impossible to ignore errors caused by the high-order components when number of interpolation is increased. Japanese Patent Laid-Open No. 03-048122 discloses an optical, encoder that reduces higher-order components included in a periodic signal. In this encoder, a reference pattern and another pattern having a phase shifted from that of the reference pattern toy ⅙ are provided mutually adjacently in a direction orthogonal, to a direction (hereinafter referred to as “a period direction”) along the period of the periodic pattern. The encoder removes a third-order component as the higher-order component by causing a light receiver to collectively receive lights transmitted through these two patterns.
However, in general optical encoders, the scale and the sensor are attached to separate members constituting an apparatus in which the encoder is provided, and thus mechanical backlash of these members may relatively displace the scale and the sensor in a direction orthogonal to the period direction of the periodic pattern. In the encoder disclosed in Japanese Patent Laid-Open No, 03-048122, a relative displacement, of the scale and the light receiver in the direction orthogonal to the period direction changes a ratio of the above-mentioned two patterns in a detection area by the light receiver on the scale. Such change of the ratio of the two patterns results in change of a phase of a fundamental wave of the periodic signal or decrease of the higher-order component removal effect, which makes it impossible to position detection with good accuracy.